Field of the Invention
The present invention relates to a resistance heating element, a fixing device, and an image forming apparatus.
Description of the Related Art
In many conventional image forming apparatuses such as copying machines and laser beam printers, unfixed toner images transferred onto image holding members such as plain paper sheets after toner development are heated and fixed in a contact manner by a heated roller method.
However, it is time-consuming to increase temperature to an image-fixable temperature by the heated roller method, and a large quantity of heat energy is also required. So as to shorten the time (warm-up time) between power activation and a copying start and save energy, a heated-film fixing method is often used these days.
In a fixing device (fixer) of a heated-film fixing type, a seamless fixing belt formed by stacking a release layer made of fluororesin or the like on the outer surface of a heat-resistant film made of polyimide or the like is used. In such a fixing device of a heated-film fixing type, a film is heated via a ceramic heater, for example, and a toner image is fixed onto the surface of the film. Therefore, the heat conductivity of the film is a critical aspect. However, if the fixing belt film is made thinner so as to improve heat conductivity, mechanical strength decreases, and high-speed rotation becomes difficult, causing a problem in forming high-quality images at high speed. Furthermore, a ceramic heater or the like is easily damaged.
So as to solve such problems, there is a recently suggested method of forming a resistance heating element on a fixing belt, directly heating the fixing belt by feeding power to the heating element, and fixing a toner image. An image forming apparatus implementing this method has a short warm-up time, and consumes less power than an image forming apparatus of a heated-film fixing type. Therefore, an image forming apparatus implementing this method excels as a heat fixing device in terms of energy saving and high-speed operation.
An example of a heating belt used in a fixing belt is disclosed in JP 2012-8299 A. According to JP 2012-8299 A, the heat-resistant resin used in the heating belt contains fibrous filler that satisfies specific requirements.
According to the technique disclosed in JP 2012-8299 A, the initial resistance value is sufficiently low, but there are cases where an increase is observed in resistance with time. If the resistance of a resistance heating element increases with time, unevenness in fixing due to defective toner fixing might be observed in a case where the resistance heating element is used in the fixing belt of an image forming apparatus.